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Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films
Materials Letters 63 (2009) 1740–1742
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films Shamshad A. Khan ⁎,1, A. A. Al-Ghamdi Department of Physics, Faculty of Science, King Abdul Aziz University, Jeddah-21589, Kingdom of Saudi Arabia
a r t i c l e
i n f o
Article history: Received 24 December 2008 Accepted 12 May 2009 Available online 18 May 2009 Keywords: Optical materials and properties Semiconductors Thin films Laser-irradiation Absorption coefficient
a b s t r a c t Amorphous thin films of glassy alloys of Se75S25 − xCdx (x = 2, 4 and 6) were prepared by thermal evaporation onto chemically cleaned glass substrates. Optical absorption and reflection measurements were carried out on as-deposited and laser-irradiated thin films in the wavelength region of 500–1000 nm. Analysis of the optical absorption data shows that the rule of no-direct transitions predominates. The laser-irradiated Se75S25 − xCdx films showed an increase in the optical band gap and absorption coefficient with increasing the time of laser-irradiation. The results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. The value of refractive index increases decreases with increasing photon energy and also by increasing the time of laser-irradiation. With the large absorption coefficient and change in the optical band gap and refractive index by the influence of laser-irradiation, these materials may be suitable for optical disc application. © 2009 Published by Elsevier B.V.
1. Introduction
2. Experimental
Laser induced changes in chalcogenides are an object of systematic investigations with a view to better understand the mechanism of the phenomena taking place in them as well as their practical applications. Se based chalcogenide glasses in the amorphous state drew great attention. Due to its high glass forming ability, Se represents a good host matrix for the investigation of chalcogenide glasses in the bulk and thin film forms [1–3]. Development of information technology demands new optical recording materials and, therefore, good knowledge of their linear optical properties is of great interest. They possess controllable phase transformation from amorphous to crystal state, which makes them suitable for optical data storage [4]. The effect of laser-irradiation on amorphous thin films is now widely used to improve the linear optical and electrical properties. Suitable laser intensity profiles in combination with multi-pulse scanning sequence have been used to reduce the number of grain boundaries [5]. A lot of research work [6–9] is going on the effect of laser-irradiation, annealing, ultraviolet irradiation, g-irradiation etc on optical and electrical properties of amorphous thing films. The aim of the present work is to study the effect of laser-irradiation on optical constants of Se75S25 − xCdx thin films.
Se75S25 − xCdx alloys with composition x = 2, 4 and 6 were synthesized from elements of Se, S and Cd with 5N purity in evacuated (10− 3 Pa) quartz ampoules by using conventional melt quenching technique. After 14 h synthesis at 1123 K, the melt was subsequently quenched in ice water. The atomic absorption spectroscopy (AAS) and X-ray diffraction (XRD) were used to confirm composition and structure respectively, for both the alloys and the films. The X-ray diffraction patterns of as-prepared and laser-irradiated thin films were taken by using X-ray diffractometer (Philips Model PW 1710). All samples have similar patterns so we present here only the XRD of Se75S23Cd2 thin film (as-prepared and laser-irradiated thin films for 15 min), shown in Fig. Fig. 1. The absence of sharp structural peaks in as-prepared films confirms the amorphous nature and the presence of sharp structural peaks in annealed films confirms the crystalline nature of the films. Thin films of 3000 Å thickness were prepared by using an Edward Coating Unit E-306, onto chemically cleaned glass substrates on a base pressure of 10− 6 Torr. The coating was made at room temperature and the rate of coating was maintained at 5 nm/s. The thickness of the films was measured by using a quartz crystal monitor, Edward model FTM 7. Before the measurements, thin films were annealed for 1 h under vacuum at a temperature lower than Tg [6], where Tg is the glass transition temperature of the powdered sample measured by using non-isothermal DSC measurement (shown in Fig. 2). The amorphous thin films were induced by pulsed TEA N2 laser (wavelength 337.1 nm, Power 100 kW, Pulse width 1 ns) for 5, 10 and 15 min. A JASCO, V-500, UV/VIS/NIR computerized spectrophotometer is used for measuring optical absorption and reflection of amorphous and laser-irradiated thin films. A JASCO-V-500-UV/VIS/
⁎ Corresponding author. E-mail address: [email protected] (S.A. Khan). 1 Permanent address. Department of Physics, St. Andrew's P.G. College, Gorakhpur273001, U. P., India. 0167-577X/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.matlet.2009.05.025
S.A. Khan, A.A. Al-Ghamdi / Materials Letters 63 (2009) 1740–1742
1741
significant parameters in chalcogenide thin films. Therefore, an accurate measurement of the optical constants is extremely important. The optical behavior of a material is generally utilized to determine its optical constants; refractive index (n) and extinction coefficient (k). Separate determinations of n and k can be made by measuring reflectance of the same specimen. The reflectance (R) of a material with n and k is given by [9–10], h i h i 2 2 2 2 R = ðn= 1Þ + k = ðn=1Þ + k
ð1Þ
Where k is the extinction coefficient and is given by Fig. 1. X-ray pattern of Se75S23Cd2 thin films: (a) as-prepared (b) laser-irradiated film for 15 min.
NIR computerized spectrophotometer is used for measuring optical absorption and reflectance separately. In fact the “absorbance” reading (i.e. photometric value) is a measure of the amount of light absorbed by the sample under specified conditions. The Beer–Lambert Law is the basis of the quantitative of UV/visible spectroscopy. In UV/ visible spectroscopy, a spectrophotometer passes a double beam of light through a sample with a fixed path length. The spectrophotometer then monitors the absorption of light in terms of optical density at the particular thickness of the sample. The absorption has been measured in terms of optical density. Thin films of glassy alloys of Se75S25 − xCdx sample have been deposited on glass substrate. Before performing the experiments for measuring the absorbance, we have kept the samples (only glass substrate) and reference (same glass substrate) in the chamber at the appropriate sample holder and made the baseline correction. After making this baseline correction, we have now kept the samples (Thin films form) and reference (only glass substrate) in the chamber at the appropriate sample holder to measure absorbance. It will give only the absorbance by the materials. The optical absorption was measured as a function of incidence photon energy. 3. Results and discussion The interest in optical properties of chalcogenide thin films has been stimulated by their possible applications as switching elements and optical transmission media, as well as by their use as passivating materials for integrated circuits. The absorption coefficient, optical band gap, refractive index and extinction coefficient are the most
Fig. 2. DSC trace of powered Se75S21Cd4 glasses at the heating rate of 10 K/min.
k = ðαλÞ = ð4πÞ
ð2Þ
Thus using relations (1) and (2), the values of refractive index (n) and extinction coefficient (k) at different incident photon energy can be calculated. The spectral dependence of refractive indexes (n) for Se75S23Cd2: amorphous and laser-irradiated thin films is shown in Fig. 3. The value of refractive index decreases with increasing photon energy. The values of n and k with different time of laser-irradiation are given in Table Table 1. It is evident from this table that the values of n and k both increase by increasing time of laser-irradiation. The spectrum of the optical absorption coefficient was computed from the absorbance data. The absorption coefficient (α) has been obtained directly from the absorbance against wavelength curves using the relation [9–11], α = Absorbance = thickness
ð3Þ
The values of the absorption coefficient (α) for amorphous and laser-irradiated thin films of Se75S25 − xCdx are given in Table 1. It is clear from this table that the order of the absorption coefficient (α) for Se75S25 − x Cdx films is in the range ~104 cm− 1, which is consistent with the result of other workers [11]. The value of absorption coefficient (α) increases with increasing the time of laser-irradiation. The fundamental absorption edge in most amorphous semiconductors follows an exponential law. Above the exponential tail, the absorption coefficient has been reported [12] to obey the following equation: ðα ·hvÞ
1=m
= B hv − Eg
ð4Þ
Where n is the frequency of the incident beam (w = 2pn), B is a constant, Eg is optical band gap and n is an exponent, which can be assumed to have values of 1/2, 3/2, 2 and 3 depending on the nature
Fig. 3. Variation of refractive index (n) with incident photon energy (h ν) in Se75S19Cd6: amorphous and laser-irradiated thin films.
1742
S.A. Khan, A.A. Al-Ghamdi / Materials Letters 63 (2009) 1740–1742
Table 1 Optical constants (refractive index n, absorption coefficient α and optical band gap Eg) of Se75S25 − xCdx: amorphous and laser-irradiated thin films at 820 nm. Sample Se75S23Cd2 Se75S21Cd4 Se75S19Cd6
Amorphous films
After laser-irradiation (5 min)
After laser-irradiation (10 min)
After laser-irradiation (15 min)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
3.92 3.21 2.88
1.76 2.12 2.85
0.75 0.89 1.11
3.78 3.05 2.67
1.89 2.56 3.21
0.83 0.92 1.23
3.62 2.91 2.36
2.13 2.76 3.48
0.89 1.16 1.29
3.41 2.68 2.04
2.37 2.89 3.56
1.02 1.27 1.41
of electronic transition responsible for the absorption: r = 1/2 for allowed direct transition, r = 3/2 for forbidden direct transition, r = 2 for allowed indirect transition and r = 3 for forbidden indirect transition [13]. The best fit of the experimental results of both asdeposited and laser-irradiated thin films using Eq. (4), with r = 2 is shown in Fig. 4. The variation curve of (α · hn)1/2 with photon energy (hn) for amorphous and laser-irradiated films is found to be identical to that of the elemental amorphous semiconductor [14–15]. This indicates that the absorption in Se75S25 − xCdx thin films is due to nondirect transition and the values of indirect optical band gap (Eg) have been calculated by taking the intercept on the X-axis. The calculated values of Eg for Se75S25 − xCdx: amorphous and laser-irradiated thin films are given in Table 1. It is evident from this table that the value of optical band gap (Eg) increase with increasing time of laserirradiation. The increase in the optical band gap with increasing time of laser-irradiation may be due to the increase in grain size, the reduction in the disorder and decrease in density of defect states (which results in the reduction of tailing of bands) [14–15]. The increase in the optical band gap could also be discussed on the basis of density of state model proposed by Mott and Davis [16]. According to this model, the width of the localized states near the mobility edges depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as the result of an insufficient number of atoms deposited in the amorphous film [17]. The unsaturated bonds are responsible for the formation of some of the defects in the films, producing localized states in the amorphous solids. The presence of high concentration of localized states in the band structure is responsible for the increase of optical band gap in the case of the amorphous films. This increase in the band gap may also be due to the shift in Fermi level whose position is determined by the distribution of electrons over the localized states. Chalcogenide thin films always contain a high concentration of unsaturated bonds
or defects. These defects are responsible for the presence of localized states in the amorphous band gap. During crystallization, the unsaturated defects are gradually annealed out producing a large number of saturated bonds. The reduction in the number of unsaturated defects decreases the density of localized states in the band structure consequently increasing the optical band gap. After laser-irradiation on amorphous thin films of Se75S25 − x Cdx the optical band gap increases, which shows that after laser-irradiation, the amorphous films were crystallized. 4. Conclusion The optical absorption measurements on the Se75S25 − xCdx: amorphous and laser-irradiated thin films indicate that the absorption occurs due to indirect transition. The optical band gap increases on increasing the time of laser-irradiation. This may be due to the increase in the grain size, the reduction in the disorderedness of the system. This may also be due to the decrease in the density of defect states, which results in the reduction of tailing of bands. The refractive index (n) decreases with increasing photon energy for amorphous and laser-irradiated thin films. It also decreases with increasing the time of laser-irradiation. With large absorption coefficients and the change in reflection, refractive index and extinction coefficient under the influence of light enables and laser-irradiation, these materials to be used in optical data storage and other devices. Acknowledgements Thanks are due to Deanship of Scientific Research, King Abdul Aziz University, Jeddah, Saudi Arabia (Reference no.:187/428) for providing financial assistance in the form of research project. Thanks are also due to Prof. M. Husain and Prof. M. Zulfequar, Department of Physics, Jamia Millia Islamia, New Delhi, India for his useful discussion and valuable suggestions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 4. (αh ν)1/2 against photon energy in Se75S19Cd6: amorphous and laser-irradiated thin films.
Aly KA, et al. J Phys Chem Solids 2008;69:2514. Sharma Pankaj, Katyal SC. Mater Lett 2007;61:4516. Kushwaha VS, Kumar A. Mater Lett 2006;60:2148. Pamukchieva V, Szekeres A. Opt Mater 2008;30:1088. Rezek B, Stutzmann M. J Non-Cryst Solids 2000;266–269:315. Othman AA. Thin Solid Films 2006;515:1634. Salem AM, et al. Nucl Instrum Methods Phy Res B 2007;262:225. Jun Xu AA, et al. Mater Lett 2007;61:5010. Frumar Miloslav, Wagner Tomas. Cur Op Sol State Mat Sci 2003;7:117. Khan Shamshad A, Zulfequar M, Husain M. Current App Phy 2005;5:583. Majeed Khan MA, Zulfequar M, Husain M. Opt Mater 2003;22:21. Tauc J. In: Tauc J, editor. Amorphous and Liquid Semiconductors. New York: Plenum Press; 1974. p. 171. Smith RA. Phil Mag Suppl 1953;2:81. Ilyas M, Zulfequar M, Husain M. J Modern Optics 2000;47:663. Nang TT, et al. Jpn J App Phys 1976;14:849. Mott NF, Davis EA. Electronics Processes in Non-crystalline Materials. Oxford: Clarendon; 1979. p. 428. Theye ML. Proc. Vth International Conference on Amorphous and Liquid Semiconductors, vol. 1; 1973. p. 479.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films Shamshad A. Khan ⁎,1, A. A. Al-Ghamdi Department of Physics, Faculty of Science, King Abdul Aziz University, Jeddah-21589, Kingdom of Saudi Arabia
a r t i c l e
i n f o
Article history: Received 24 December 2008 Accepted 12 May 2009 Available online 18 May 2009 Keywords: Optical materials and properties Semiconductors Thin films Laser-irradiation Absorption coefficient
a b s t r a c t Amorphous thin films of glassy alloys of Se75S25 − xCdx (x = 2, 4 and 6) were prepared by thermal evaporation onto chemically cleaned glass substrates. Optical absorption and reflection measurements were carried out on as-deposited and laser-irradiated thin films in the wavelength region of 500–1000 nm. Analysis of the optical absorption data shows that the rule of no-direct transitions predominates. The laser-irradiated Se75S25 − xCdx films showed an increase in the optical band gap and absorption coefficient with increasing the time of laser-irradiation. The results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. The value of refractive index increases decreases with increasing photon energy and also by increasing the time of laser-irradiation. With the large absorption coefficient and change in the optical band gap and refractive index by the influence of laser-irradiation, these materials may be suitable for optical disc application. © 2009 Published by Elsevier B.V.
1. Introduction
2. Experimental
Laser induced changes in chalcogenides are an object of systematic investigations with a view to better understand the mechanism of the phenomena taking place in them as well as their practical applications. Se based chalcogenide glasses in the amorphous state drew great attention. Due to its high glass forming ability, Se represents a good host matrix for the investigation of chalcogenide glasses in the bulk and thin film forms [1–3]. Development of information technology demands new optical recording materials and, therefore, good knowledge of their linear optical properties is of great interest. They possess controllable phase transformation from amorphous to crystal state, which makes them suitable for optical data storage [4]. The effect of laser-irradiation on amorphous thin films is now widely used to improve the linear optical and electrical properties. Suitable laser intensity profiles in combination with multi-pulse scanning sequence have been used to reduce the number of grain boundaries [5]. A lot of research work [6–9] is going on the effect of laser-irradiation, annealing, ultraviolet irradiation, g-irradiation etc on optical and electrical properties of amorphous thing films. The aim of the present work is to study the effect of laser-irradiation on optical constants of Se75S25 − xCdx thin films.
Se75S25 − xCdx alloys with composition x = 2, 4 and 6 were synthesized from elements of Se, S and Cd with 5N purity in evacuated (10− 3 Pa) quartz ampoules by using conventional melt quenching technique. After 14 h synthesis at 1123 K, the melt was subsequently quenched in ice water. The atomic absorption spectroscopy (AAS) and X-ray diffraction (XRD) were used to confirm composition and structure respectively, for both the alloys and the films. The X-ray diffraction patterns of as-prepared and laser-irradiated thin films were taken by using X-ray diffractometer (Philips Model PW 1710). All samples have similar patterns so we present here only the XRD of Se75S23Cd2 thin film (as-prepared and laser-irradiated thin films for 15 min), shown in Fig. Fig. 1. The absence of sharp structural peaks in as-prepared films confirms the amorphous nature and the presence of sharp structural peaks in annealed films confirms the crystalline nature of the films. Thin films of 3000 Å thickness were prepared by using an Edward Coating Unit E-306, onto chemically cleaned glass substrates on a base pressure of 10− 6 Torr. The coating was made at room temperature and the rate of coating was maintained at 5 nm/s. The thickness of the films was measured by using a quartz crystal monitor, Edward model FTM 7. Before the measurements, thin films were annealed for 1 h under vacuum at a temperature lower than Tg [6], where Tg is the glass transition temperature of the powdered sample measured by using non-isothermal DSC measurement (shown in Fig. 2). The amorphous thin films were induced by pulsed TEA N2 laser (wavelength 337.1 nm, Power 100 kW, Pulse width 1 ns) for 5, 10 and 15 min. A JASCO, V-500, UV/VIS/NIR computerized spectrophotometer is used for measuring optical absorption and reflection of amorphous and laser-irradiated thin films. A JASCO-V-500-UV/VIS/
⁎ Corresponding author. E-mail address: [email protected] (S.A. Khan). 1 Permanent address. Department of Physics, St. Andrew's P.G. College, Gorakhpur273001, U. P., India. 0167-577X/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.matlet.2009.05.025
S.A. Khan, A.A. Al-Ghamdi / Materials Letters 63 (2009) 1740–1742
1741
significant parameters in chalcogenide thin films. Therefore, an accurate measurement of the optical constants is extremely important. The optical behavior of a material is generally utilized to determine its optical constants; refractive index (n) and extinction coefficient (k). Separate determinations of n and k can be made by measuring reflectance of the same specimen. The reflectance (R) of a material with n and k is given by [9–10], h i h i 2 2 2 2 R = ðn= 1Þ + k = ðn=1Þ + k
ð1Þ
Where k is the extinction coefficient and is given by Fig. 1. X-ray pattern of Se75S23Cd2 thin films: (a) as-prepared (b) laser-irradiated film for 15 min.
NIR computerized spectrophotometer is used for measuring optical absorption and reflectance separately. In fact the “absorbance” reading (i.e. photometric value) is a measure of the amount of light absorbed by the sample under specified conditions. The Beer–Lambert Law is the basis of the quantitative of UV/visible spectroscopy. In UV/ visible spectroscopy, a spectrophotometer passes a double beam of light through a sample with a fixed path length. The spectrophotometer then monitors the absorption of light in terms of optical density at the particular thickness of the sample. The absorption has been measured in terms of optical density. Thin films of glassy alloys of Se75S25 − xCdx sample have been deposited on glass substrate. Before performing the experiments for measuring the absorbance, we have kept the samples (only glass substrate) and reference (same glass substrate) in the chamber at the appropriate sample holder and made the baseline correction. After making this baseline correction, we have now kept the samples (Thin films form) and reference (only glass substrate) in the chamber at the appropriate sample holder to measure absorbance. It will give only the absorbance by the materials. The optical absorption was measured as a function of incidence photon energy. 3. Results and discussion The interest in optical properties of chalcogenide thin films has been stimulated by their possible applications as switching elements and optical transmission media, as well as by their use as passivating materials for integrated circuits. The absorption coefficient, optical band gap, refractive index and extinction coefficient are the most
Fig. 2. DSC trace of powered Se75S21Cd4 glasses at the heating rate of 10 K/min.
k = ðαλÞ = ð4πÞ
ð2Þ
Thus using relations (1) and (2), the values of refractive index (n) and extinction coefficient (k) at different incident photon energy can be calculated. The spectral dependence of refractive indexes (n) for Se75S23Cd2: amorphous and laser-irradiated thin films is shown in Fig. 3. The value of refractive index decreases with increasing photon energy. The values of n and k with different time of laser-irradiation are given in Table Table 1. It is evident from this table that the values of n and k both increase by increasing time of laser-irradiation. The spectrum of the optical absorption coefficient was computed from the absorbance data. The absorption coefficient (α) has been obtained directly from the absorbance against wavelength curves using the relation [9–11], α = Absorbance = thickness
ð3Þ
The values of the absorption coefficient (α) for amorphous and laser-irradiated thin films of Se75S25 − xCdx are given in Table 1. It is clear from this table that the order of the absorption coefficient (α) for Se75S25 − x Cdx films is in the range ~104 cm− 1, which is consistent with the result of other workers [11]. The value of absorption coefficient (α) increases with increasing the time of laser-irradiation. The fundamental absorption edge in most amorphous semiconductors follows an exponential law. Above the exponential tail, the absorption coefficient has been reported [12] to obey the following equation: ðα ·hvÞ
1=m
= B hv − Eg
ð4Þ
Where n is the frequency of the incident beam (w = 2pn), B is a constant, Eg is optical band gap and n is an exponent, which can be assumed to have values of 1/2, 3/2, 2 and 3 depending on the nature
Fig. 3. Variation of refractive index (n) with incident photon energy (h ν) in Se75S19Cd6: amorphous and laser-irradiated thin films.
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S.A. Khan, A.A. Al-Ghamdi / Materials Letters 63 (2009) 1740–1742
Table 1 Optical constants (refractive index n, absorption coefficient α and optical band gap Eg) of Se75S25 − xCdx: amorphous and laser-irradiated thin films at 820 nm. Sample Se75S23Cd2 Se75S21Cd4 Se75S19Cd6
Amorphous films
After laser-irradiation (5 min)
After laser-irradiation (10 min)
After laser-irradiation (15 min)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
n
α (104) (cm− 1)
Eg (eV)
3.92 3.21 2.88
1.76 2.12 2.85
0.75 0.89 1.11
3.78 3.05 2.67
1.89 2.56 3.21
0.83 0.92 1.23
3.62 2.91 2.36
2.13 2.76 3.48
0.89 1.16 1.29
3.41 2.68 2.04
2.37 2.89 3.56
1.02 1.27 1.41
of electronic transition responsible for the absorption: r = 1/2 for allowed direct transition, r = 3/2 for forbidden direct transition, r = 2 for allowed indirect transition and r = 3 for forbidden indirect transition [13]. The best fit of the experimental results of both asdeposited and laser-irradiated thin films using Eq. (4), with r = 2 is shown in Fig. 4. The variation curve of (α · hn)1/2 with photon energy (hn) for amorphous and laser-irradiated films is found to be identical to that of the elemental amorphous semiconductor [14–15]. This indicates that the absorption in Se75S25 − xCdx thin films is due to nondirect transition and the values of indirect optical band gap (Eg) have been calculated by taking the intercept on the X-axis. The calculated values of Eg for Se75S25 − xCdx: amorphous and laser-irradiated thin films are given in Table 1. It is evident from this table that the value of optical band gap (Eg) increase with increasing time of laserirradiation. The increase in the optical band gap with increasing time of laser-irradiation may be due to the increase in grain size, the reduction in the disorder and decrease in density of defect states (which results in the reduction of tailing of bands) [14–15]. The increase in the optical band gap could also be discussed on the basis of density of state model proposed by Mott and Davis [16]. According to this model, the width of the localized states near the mobility edges depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as the result of an insufficient number of atoms deposited in the amorphous film [17]. The unsaturated bonds are responsible for the formation of some of the defects in the films, producing localized states in the amorphous solids. The presence of high concentration of localized states in the band structure is responsible for the increase of optical band gap in the case of the amorphous films. This increase in the band gap may also be due to the shift in Fermi level whose position is determined by the distribution of electrons over the localized states. Chalcogenide thin films always contain a high concentration of unsaturated bonds
or defects. These defects are responsible for the presence of localized states in the amorphous band gap. During crystallization, the unsaturated defects are gradually annealed out producing a large number of saturated bonds. The reduction in the number of unsaturated defects decreases the density of localized states in the band structure consequently increasing the optical band gap. After laser-irradiation on amorphous thin films of Se75S25 − x Cdx the optical band gap increases, which shows that after laser-irradiation, the amorphous films were crystallized. 4. Conclusion The optical absorption measurements on the Se75S25 − xCdx: amorphous and laser-irradiated thin films indicate that the absorption occurs due to indirect transition. The optical band gap increases on increasing the time of laser-irradiation. This may be due to the increase in the grain size, the reduction in the disorderedness of the system. This may also be due to the decrease in the density of defect states, which results in the reduction of tailing of bands. The refractive index (n) decreases with increasing photon energy for amorphous and laser-irradiated thin films. It also decreases with increasing the time of laser-irradiation. With large absorption coefficients and the change in reflection, refractive index and extinction coefficient under the influence of light enables and laser-irradiation, these materials to be used in optical data storage and other devices. Acknowledgements Thanks are due to Deanship of Scientific Research, King Abdul Aziz University, Jeddah, Saudi Arabia (Reference no.:187/428) for providing financial assistance in the form of research project. Thanks are also due to Prof. M. Husain and Prof. M. Zulfequar, Department of Physics, Jamia Millia Islamia, New Delhi, India for his useful discussion and valuable suggestions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 4. (αh ν)1/2 against photon energy in Se75S19Cd6: amorphous and laser-irradiated thin films.
Aly KA, et al. J Phys Chem Solids 2008;69:2514. Sharma Pankaj, Katyal SC. Mater Lett 2007;61:4516. Kushwaha VS, Kumar A. Mater Lett 2006;60:2148. Pamukchieva V, Szekeres A. Opt Mater 2008;30:1088. Rezek B, Stutzmann M. J Non-Cryst Solids 2000;266–269:315. Othman AA. Thin Solid Films 2006;515:1634. Salem AM, et al. Nucl Instrum Methods Phy Res B 2007;262:225. Jun Xu AA, et al. Mater Lett 2007;61:5010. Frumar Miloslav, Wagner Tomas. Cur Op Sol State Mat Sci 2003;7:117. Khan Shamshad A, Zulfequar M, Husain M. Current App Phy 2005;5:583. Majeed Khan MA, Zulfequar M, Husain M. Opt Mater 2003;22:21. Tauc J. In: Tauc J, editor. Amorphous and Liquid Semiconductors. New York: Plenum Press; 1974. p. 171. Smith RA. Phil Mag Suppl 1953;2:81. Ilyas M, Zulfequar M, Husain M. J Modern Optics 2000;47:663. Nang TT, et al. Jpn J App Phys 1976;14:849. Mott NF, Davis EA. Electronics Processes in Non-crystalline Materials. Oxford: Clarendon; 1979. p. 428. Theye ML. Proc. Vth International Conference on Amorphous and Liquid Semiconductors, vol. 1; 1973. p. 479.